We investigate the causes of large $dB/dt$ events observed by SuperMAG, by comparing with the time-series of different types of geomagnetic activity, or “convection state”, for the duration of 2010. Spikes are found to occur predominantly in the pre-midnight and dawn sectors. We find that pre-midnight spikes are associated with substorm onsets. Dawn sector spikes are not directly associated with substorms, but with auroral activity occurring within the westward electrojet region. Azimuthally-spaced auroral features drift sunwards, producing Ps6 (10-20 min period) magnetic perturbations on the ground. The magnitude of $dB/dt$ is determined by the flow speed in the convection return flow region, which in turn is related to the strength of solar wind-magnetospheric coupling. Pre-midnight and dawn sector spikes can occur at the same time, as strong coupling favours both substorms and westward electrojet activity; however, the mechanisms that create them seem somewhat independent. The dawn auroral features share some characteristics with omega bands, but can also appear as north-south aligned auroral streamers. We suggest that these two phenomena share a single underlying cause.

In theory the width of the quasi-perpendicular bow shock ramp is on the scale of a few electron inertial lengths, but as this work will show the quasi-perpendicular bow shock at Mars is often wider. This is important because it implies that the conditions at Mars create a behaviour at the shock which cannot be described by current theory. Furthermore, the width could affect processes at the shock such as energy transfer of the ions and their subsequent thermalization. To investigate the cause of the width, two sets of quasi-perpendicular bow shock crossings measured by MAVEN are compared, one of unusual width (average 370 km or 5r$_{gi}$), and one of typical width (average 30 km or 0.7r$_{gi}$). These sets are labeled wide and thin shocks respectively. It is seen that the wide shocks have no distinct overshoot and have a higher level of magnetic field fluctuations than the thin shocks. Factors that are known to affect the standoff distance, such as the magnetosonic Mach number and mass loading of the solar wind by planetary species, were found not to affect the width of the bow shock. It is found that the temperature of the solar wind plasma increases more as it passes through a wide than a thin shock, indicating that ions are thermalized to a larger extent than at thin shocks. The larger-than-predicted by theory width of the Martian quasi-perpendicular bow shock indicate that there are conditions at Mars which we do not yet understand.

Rapid changes of magnetic fields associated with nighttime magnetic perturbation events (MPEs) with amplitudes |ΔB| of hundreds of nT and 5-10 min periods can induce geomagnetically-induced currents (GICs) that can harm technological systems. In this study we compare the occurrence and amplitude of nighttime MPEs with |dB/dt| ≥ 6 nT/s observed during 2015 and 2017 at five stations in Arctic Canada ranging from 75.2° to 64.7° in corrected geomagnetic latitude (MLAT) as functions of magnetic local time (MLT), the SME and SYM/H magnetic indices, and time delay after substorm onsets. Although most MPEs occurred within 30 minutes after a substorm onset, ~10% of those observed at the four lower latitude stations occurred over two hours after the most recent onset. A broad distribution in local time appeared at all 5 stations between 1700 and 0100 MLT, and a narrower distribution appeared at the lower latitude stations between 0200 and 0700 MLT. There was little or no correlation between MPE amplitude and the SYM/H index; most MPEs at all stations occurred for SYM/H values between -40 and 0 nT. SME index values for MPEs observed more than 1 hour after the most recent substorm onset fell in the lower half of the range of SME values for events during substorms, and dipolarizations in synchronous orbit at GOES 13 during these events were weaker or more often nonexistent. These observations suggest that substorms are neither necessary nor sufficient to cause MPEs, and hence predictions of GICs cannot focus solely on substorms.

Nearly all studies of impulsive magnetic perturbation events (MPEs) with large magnetic field variability (dB/dt) that can produce dangerous geomagnetically-induced currents (GICs) have used data from the northern hemisphere. Here we present details of four large-amplitude MPE events (|DBx|> 900 nT and |dB/dt| > 10 nT/s in at least one component) observed between 2015 and 2018 in conjugate high latitude regions (65 - 80° corrected geomagnetic latitude), using magnetometer data from (1) Pangnirtung and Iqaluit in eastern Arctic Canada and the magnetically conjugate South Pole Station in Antarctica and (2) the Greenland West Coast Chain and two magnetically conjugate chains in Antarctica, AAL-PIP and BAS LPM. From 1 to 3 different isolated MPEs localized in corrected geomagnetic latitude were observed during 3 pre-midnight events; many were simultaneous within 3 min in both hemispheres. Their conjugate latitudinal amplitude profiles, however, matched qualitatively at best. During an extended post-midnight interval, which we associate with an interval of omega bands, multiple highly localized MPEs occurred independently in time at each station in both hemispheres. These nighttime MPEs occurred under a wide range of geomagnetic conditions, but common to each was a negative IMF Bz that exhibited at least a modest increase at or near the time of the event. A comparison of perturbation amplitudes to modeled ionospheric conductivities in conjugate hemispheres clearly favored a current generator model over a voltage generator model for 3 of the 4 events; neither model provided a good fit for the pre-midnight event that occurred near vernal equinox.

On 19 October 2014, Mars experienced a very close encounter with Comet C/2013 A1 Siding Spring. Using data from the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) on board Mars Express (MEX), we assess the interaction of the Martian ionosphere with the comet’s atmosphere and possibly magnetic tail during the orbit of their closest approach. The topside ionospheric electron density profile is evaluated from the peak density of the ionosphere to the local plasma around Mars Express. We find unusual, complex and rapid variability in the ionospheric profile along the MEX orbit, not seen even after the impact of large coronal mass ejections. Before closest approach, large electron density reductions predominate, which could be caused either by comet water-damping, or comet magnetic field interactions. After closest approach, a substantial electron density rise predominates. Moreover, several extra topside layers are visible along the whole orbit at different altitudes, which could be related to different processes as we discuss.

Geomagnetically induced currents or GICs are signatures of a rapidly time-varying magnetic field (dB/dt) and occur mainly during substorms and storms. When, where and why exactly GICs may occur, is still vague. Thus, we investigated storms for the last 40 years (from 1980 with a storm-list created by W.T. Walach) and analyzed the negative and positive dB/dt spikes (threshold of 500 nT/min) in the north and east component using a worldwide coverage (SuperMAG). Our analysis confirmed the existence of two dB/dt spikes “hotspots” located in the pre-midnight and in the morning MLT sector, independently of the geographic location of the stations. The associated physical ionospheric phenomena are most probably substorm current wedge (SCW) onsets and westward travelling surges (WTS) in the evening sector, and wave- or vortex-like current flows in Omega bands in the morning sector. Additionally, we observed a spatio-temporal evolution of the negative northern dB/dt spikes. The spikes initially occur in the pre-midnight sector, and then develop in time towards the morning sector. This spatio-temporal sequence is correlated with bursts in the AE index, and can be repeated several times throughout a storm. Finally, we investigated the intensity (Dst and AE) of the storms compared to the number of dB/dt spikes, but we did not find any correlation. This result implies that moderate storm with many spikes can be as (or more) dangerous for ground-based infrastructures than a major storm with fewer dB/dt spikes. Our findings may help to improve the GICs forecast to accurately predict dB/dt spikes.